The present invention relates to systems for cooling which employ a helical screw type compressor. More specifically, the present invention relates to unloading of a multi-screw compressor by shifting or displacing at least one of the rotors.
Cooling systems in the HVAC (heating, ventilation and air conditioning) industry are well known. By way of example, a schematic diagram of a typical cooling system is shown in FIG. 1 herein, labeled prior art. Referring to FIG. 1 herein, water enters an evaporator 412 through an input 414 where it is circulated through tubes within the evaporator and exits through an output 416. Liquid phase refrigerant enters evaporator 412 at an input 420 and evaporated refrigerant is delivered to a compressor 422 (e.g., a helical twin screw type compressor, which are well known in the art). Compressed vapor phase refrigerant is passed through an oil separator 424 for removing oil picked up in compressor 422. Thereafter the compressed vapor phase refrigerant is presented to a water cooled condenser 426 to condense the refrigerant to the liquid phase which is used for cooling, as is well known in the art. It will also be appreciated that air cooled condensers are well known and such could be used in place of the aforementioned water cooled condenser. Thereafter, liquid phase refrigerant is presented to an economizer 428 where vapor phase refrigerant (it is well known that a small portion of the refrigerant will be vapor, i.e., flash gas) is drawn off and delivered directly to the compressor. The liquid phase refrigerant is presented to input 420 of evaporator 412, thereby completing the cycle. When capacity of such a system is to be varied, it is common to unload the compressor, however, this is both inefficient and invariably, seriously complicates the overall design/cost of the compressor.
Further, helical type compressors are well known in the art. One such helical compressor employs one male rotor axially aligned (i.e., axially parallel) with and in communication with one female rotor. The pitch diameter of the female rotor is usually greater than the pitch diameter of the male rotor. Typically, the male rotor is the drive rotor, however compressors have been built with the female rotor being the drive rotor. The combination of one male rotor and one female rotor in a compressor is commonly referred to as a twin screw or rotor, such is well know in the art and has been in commercial use for decades. An example of one such twin rotor commonly employed with compressors in the HVAC (heating, ventilation and air conditioning) industry comprises a male rotor which drives an axially aligned female rotor. A resulting gap between the male and female rotors requires oil to be introduced into the compression area for sealing, however, the oil also provides cooling and lubricating, as is well known. However, the introduction of this oil requires the use of an oil separation device, to separate the oil from the refrigerant being compressed in HVAC compressors. A primary benefit of the twin rotor configuration is the low interface velocity between the male and female rotors during operation. However, the twin rotor configuration incurs large radial bearing loads and thrust loads. The obvious solution to alleviating the bearing load problem would be to install sufficiently sized bearings. This is not a feasible solution, since the relative diameters of the rotors in practice result in the rotors being too close together to allow installation of sufficiently sized bearings.
The prior art has addressed this problem, with the introduction compressors employing `so-called` single screw technology. A single screw configuration comprises a drive rotor with two opposing axially perpendicular gate rotors. The gate rotors are generally comprised of a composite material which allows positioning of the gate rotor with small clearances from the drive rotor. These clearances are small enough that the liquid refrigerant itself provides sufficient sealing, the liquid refrigerant also provides cooling and lubrication. The rearward positioning of gate rotors and the positioning on opposing sides of the drive rotor, (1) allows equalizing suction of pressure at both ends of the drive rotor thereby virtually eliminating the thrust loads encountered with the above described twin screw system and (2) balances the radial loading on the drive rotor thereby minimizing radial bearing loads. However, the interface velocity between the gate rotors and the drive rotor are very high. Accordingly, a common problem with this system is the extensive damage suffered by the rotors when lubrication is lost, due to the high interface velocities of the rotors.